Measurement of serum creatinine--current status and future goals

Abstract

The first methods for the measurement of creatinine in serum and plasma were published over a century ago. Today, the Jaffe reaction using alkaline picrate remains the cornerstone of most current routine methods, after continuous refinements attempting to overcome inherent analytical interferences and limitations. With the recent introduction of the reporting of estimated glomerular filtration rate (eGFR), inter-laboratory agreement of serum creatinine results has become an important international priority. Expert professional bodies have recommended that all creatinine methods should become traceable to a reference method based on isotope dilution-mass spectrometry (IDMS). It is important that clinical biochemists have a good understanding of the relative performance of routine creatinine methods. Using a new commutable IDMS-traceable reference material (SRM 967), and a validated tandem IDMS assay developed in our laboratory, we assessed the accuracy of nine routine creatinine methods with assistance from other laboratories in our region. Three methods appeared to have patient sample bias that exceeded 5% in the range of creatinine concentrations where eGFR estimations are most important. Companies are currently recalibrating their creatinine assays. This task should be complete in 2007, and then creatinine results for eGFR calculations will require the use of a modified eGFR equation. Laboratories considering calibration changes before this time can seek advice from the Australasian Creatinine Working Group.

Figure 1

Absolute difference in results between two Roche IDMS–aligned creatinine assays (rate-blanked Jaffe assay with offset minus enzymatic Creatinine Plus assay) compared to creatinine concentration. The lithium-heparin plasma samples were prepared from blood donors (“normal” samples) and pre- and post-dialysis patients. Reproduced with permission from the Editor of Clinical Laboratory (Clin Lab 2000;46:589–90)

Figure 2

a) Median percent bias (with 95% confidence intervals) in creatinine measurement for 16 serum samples with creatinine ≤ 150 μmol/L, analysed by 9 instrument/methods in 6 different laboratories. The region for acceptable method bias is shaded; b) Median percent bias (with 95% confidence intervals) in creatinine measurement for 15 serum samples with creatinine >150 μmol/L analysed by the same systems.

Figure 3

Estimated GFR values, calculated from creatinine measurement of 31 serum samples analysed by 9 instrument/methods in 6 different laboratories, compared to eGFR values from the equation shown for the IDMS method.